Thermal printers are commonly used to print alphanumeric characters and bar codes on a variety of printing media such as paper, label stock, tubing, etc. Thermal printers utilize a thermal printhead having a line of thermal printing elements, each of which may be selectively heated. As each printing element is heated, appropriate markings are applied to the printing media, either directly or through a meltable transfer medium.
The thermal printheads used in thermal printers generally include both mechanical components containing the printing elements and associated electrical circuitry applying heating signals to the printing elements. The mechanical printhead is generally formed by a fairly thick substrate of aluminum or some other material that conducts heat readily. A ceramic insulating layer having a high thermal conductivity is then formed on the upper surface of the aluminum. The insulating layer preferably not only conducts heat well, but it also has a relatively low heat capacity so that it does not itself retain heat transferred to the substrate. A relatively thin underglaze layer coats the insulating layer, and a metallic pattern is then placed on top of the underglaze layer to form the conductors for the printing elements. The conductive pattern may include an elongated anode conductor extending along the length of the printhead, and a plurality of spaced-apart finger conductors projecting perpendicularly from the elongated anode. Individual conductive leads are interleaved with the finger conductors. A bar of resistive material overlies the finger conductors and individual leads so that current will flow through the resistive material from a finger conductor to any individual lead that is connected to ground. Thus, a "dot" of resistive material can be heated by simply grounding an individual lead positioned between two finger conductors. The length of the dot corresponds to the distance between adjacent finger conductors. An electrically insulative but thermally conductive overglaze is then placed over the resistive material and conductors.
The above-described structure is used for a thick film printhead. A thin film printhead has substantially the same structure except that the individual leads are generally positioned adjacent a projecting finger conductor rather than between two finger conductors. A resistive sheet overlies the finger conductors and individual leads so that localized "dots" of the resistive sheet may be heated by selectively grounding the individual leads.
The electrical components of the printhead generally include a set of registers which receive a serial data stream of data bits corresponding in number to the number of printing elements. The registers retain the data bits and ground the individual leads corresponding to the registers that store a logical "1". However, the data output by each register is generally ANDed with a strobe signal to precisely control the timing and duration of the grounding of the individual leads.
One important limitation on the operating capability of thermal printers is their printing speed. The printing speed of a thermal printhead is limited by the time required to heat a printing element to an appropriate temperature in order to form a mark on a printing medium, as well as the time required for the printing element to cool so that a mark is not formed on the printing medium when no mark is desired. The time required to heat the printing element is a function of the current flowing through the resistive bar or sheet between conductors. The time required for a printing element to cool is a function of the thermal conductivity from the printing element to the substrate. While the print speed can be improved by using thin film printhead technology having a lower thermal mass, it would nevertheless be desirable to increase the speed of thermal printers.
Another problem with conventional thermal printers is that they lack the capability to perform various printing functions that are available with other types of printers. For example, thermal printers generally are incapable of performing high quality "gray scale" printing. For this reason, the use of thermal printers has generally been limited to printing alphanumeric letters, bar codes, and the like. Similarly, the resolution of conventional thermal printers is generally set at a fixed value, such as 150 dots per inch ("DPI"), and this fixed resolution cannot be varied without changing the printhead. However, different types of printing needs often require different printing resolutions. It would therefore be desirable to have a thermal printer that could provide the relatively high speed and low data requirement capabilities of a low resolution printhead yet also be able to provide the high quality printing capabilities of a high resolution printhead.
Another limitation of conventional thermal printers is their inability to alter the shape or aspect ratio of their printing elements. As explained above, the shape of the printing element is determined by the physical structure and geometry of the conductive pattern and overlying resistive layer. While different printing element shapes and aspect ratios can be achieved with different physical designs, the shape and aspect ratio of the printing element is nevertheless fixed for any particular design.
Another problem that sometimes occurs with conventional thermal printers results from changes in the resistivity of the resistive coating either with age or as a result of a malfunction. If the resistivity of some printing elements changes more than the resistivity of other printing elements, then the image formed on the printing medium will not have a uniform print density. If the resistance increases significantly, the printing element may even become unusable.
While thermal printers have found common acceptance, the above-described problems have nevertheless limited their usefulness for certain printing needs where optimum print quality, speed, and/or capabilities are required.